Cobalt base hard surfacing alloy



United States Patent COBALT BASE HARD SURFACING ALLOY Grant Hansel, Jr., West Winfield, N. Y., assignor to General Electric Company, a corporation of New York N 0 Drawing. Application December 26, 1956 Serial No. 630,477

4 Claims. (Cl. 75--171) My invention relates to new and useful alloys. More particularly the invention relates to alloys which can be drawn into wire form for use in continuous feed equipment and are of a composition which permits their use in the preparation of hard, erosionand corrosion-resisting surfaces.

Hard erosionand corrosion-resisting alloys which can be used to surface areas of mechanical equipment subjected to excessive local abrasive wear'are well known. Hard surfacing is often used to protect the valve seats of internal combustion engines against excessive wear. The use of hard, surfacing alloy to protectthe leading edges of steam turbine blades or buckets, particularly in the last stages, is also well known. The erosion or wear from which steam turbine blades must be shielded in this latter instance, is caused primarily by high speed collisions between the blade and droplets of condensed steam. With the outer portions of the blades or buckets moving at velocities of over 1000 feet per second in modern turbines, the wearing :of unprotected collision areas is so excessive that reblading is often necessary after only a few months of operation. As pointed out above, a solution to this problem of excessive wear lies in surfacing the leading edges of the blades or buckets with an erosionand corrosion-resisting alloy. Prior to this invention, steam turbine buckets were typically protected from erosion by high speed collision with water droplets by brazing alloy inserts into suitable recesses machined into the blade. Providing an erosion shield by welding the molten metal directly to the blade has been found unsatisfactory because it results in a dynamically unsound bucket and .because excessive heating of the blade occurs. In a more recent innovation, the metal is sprayed directly onto the bucket by metal spraying techniques whose fundamentals are well known. A production metal spraying operation requires that the metal be in the form of a continuous wire so that it may be fed to the metal spraying equipment automatically. While some of the prior art hard surfacing alloys available in wire form are quite suitable in certain applications as to their wear resisting qualities, they have been found unable to provide the type of erosion resistance needed for turbine bucket erosion shields. On the other hand, prior art compositions which provide the necessary type of erosion resistance cannot be drawn into wire form :on a commercial basis. Because of the extreme ditliculty with which they are formed, these compositions have been made available almost exclusively in cast form. Such prior alloys, in their available form, are in short lengths and contain shrinkage cracks which, during metal spraying, cause the periodic formation of large particles of molten metal which are uncontrolled in flight when sprayed and have been known to burn holes in an operators face shield and clothing. Further, since metal spraying equipment must be reloaded while a portion of the cast length of wire still remains in the metal spraying gun and since prior art alloys may be obtained only as cast rods of only l atented Oct. 7, a

several feet in length, losses due to unused wire left in the equipment are excessive.

An object of this invention is to provide a hard surfacing erosionand corrosion-resisting alloy which may be readily drawn into small diameter wire of long lengths for use in automatic equipment or which, if desired, may be used in shorter lengths.

Briefly stated, my invention relates to alloys comprising cobalt, chromium, tungsten, nickel, manganese, and carbon, along with minor amounts of other elements. The composition of my new alloy and the method of its preparation are such that it may be drawn into small diameter wire of relatively long lengths and be production metal sprayed in a hazard-free manner to produce a surface of high erosion and corrosion resistance.

Those features of my invention which I believe to be novel are set forth with particularity in the claims appended hereto. My invention, however, along with further objects and advantages thereof, will be better understood from a consideration of the following description.

I have found that an alloy consisting of, by weight, a

minimum of 56 percent cobalt, 27-30 percent chromium,

3-6 percent tungsten, 2-4 percent nickel, from 0.03-0.20 percent manganese, and from 0.25 to 0.40 percent carbon with the remainder of other elements up to a maximum of 2 percent is very suitable as an erosionand corrosionresistant alloy. Furthermore, such an alloy is readily drawn into wire for use in automatic metal spraying and is free from shrinkage cracks and other defects. While alloys of my invention made within the above prescribed limits are suitable for the purposes described, the preferred composition of my invention consists of 28 percent chromium, 4.5 percent tungsten, 2 percent nickel, 0.15 percent manganese, 0.300.35 percent carbon, with the balance essentially cobalt.

It is believed that the desirable properties of my alloys with respect to impact erosion resistance are primarily due to the presence of a hard, homogeneous, solid solution of chromium and tungsten or their carbides in cobalt. It will be noted that the carbon content has been substantially reduced as compared with most other alloys of this general type. Carbon contents in excess of the amount soluble in the solid solution of my material form brittle carbides which have been found to reduce the hot workability of the alloy to the extent that wire drawing becomes impossible. Reduction of the carbon content to permit such hot working has been found not to detract from the other characteristics of the alloy, particularly with respect to impact erosion resistance.

The addition of nickel to the cobalt-chromium alloy base has been found to impart ductility to and enhance the hot workability of the alloy. Although prior art alloys containing nickel in excess of 10 percent may be hot worked into the wire form desired, the specific properties of my invention as evaluated by impact erosion tests show that such high nickel contents yield inferior resistance to impact erosion. Manganese is well known as a sulfur scavenger. Since cobalt becomes increasingly brittle in the presence of its sulfide, the manganese addition further contributes to the hot working characteristics of the alloy. Tungsten, like chromium, contributes to the erosion-resistant properties of the solid solution, and this highly dense element, when alloyed with a cobaltchromium base, exhibits impact erosion resistance far superior to that which is shown by an alloy in which molybdenum is used in lieu of tungsten.

In addition to the ductility and hot workability imparted to the alloy of the invention by its composition, it has been found that such characteristics are further enhanced by vacuum melting the constituents in the well known manner.

In a typical procedure according to my invention, after vacuum melting and casting into ingots of suitable size, bars approximately one-inch square are obtained by forging at an initial or primary metal temperature of about 2150-2225 F. The bars are then reheated to about 21502235 F. and reduced to a diameter of one-quarter inch by rolling, with the alloy being sufficiently ductile to permit final rolling passes to be conducted at temperatures below about 1300" F. It is unnecessary to interrupt this rolling operation to reheat the alloy. Swaging is conducted at about 2100 F. until a suitable size has been attained for drawing into the dsired wire size of the order of one-eighth inch diameter. Wire drawing is carried out in the usual manner at about 1400-1600 F. It will be realized, of course, that the above data as regards temperature and size of bars may be varied widely depending upon the specific final characteristics desired in the alloys, and these factors will be varied readily by 4 those skilled in the art.

The following examples of alloy compositions are exemplary of those of my invention. All percentages are by weight.

Nickel Manganese Chro- 'Iung- Carbon Other mium sten Ex. Cobalt Pe'cent Percent Percent Percent Percent Percent Percent 1.

As pointed out above, primary hot working of the alloys of the above examples is preferably carried out at temperatures of about 2150 to 2250 F while wire drawing is carried out at temperatures ranging from about 1400 to 1600 F.

Example 3 above as typical of those set forth, while having an as cast hardness as low as Re 32 (Rockwell C scale hardness number 32) is able to sustain the severe deformation of being made into wire, by virtue of the alloy composition and vacuum melting which are used such that an Re 54 hardness can be achieved during the wire drawing process without causing this material to undergo failure. The as sprayed hardness is of the order of Rc 42,

There is provided by this invention a hard surface material having superior erosionand corrosion-resisting properties which, in addition, is capable of being hot worked and drawn into wire of continuous lengths.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A hard surfacing metal alloy in continuous wire form comprising, by weight, from 27 to 30 percent chromium, a minimum of 56 percent cobalt, from 3 to 6 percent tungsten, from 2 to 4 percent nickel, from 0.03 to 0.2 percent manganese and from 0.25 to 0.40 percent carbon.

2. A hard surfacing metal alloy in continuous wire form comprising, by weight, 28 percent chromium, 4.5 percent tungsten, 2.0 percent nickel, 0.15 percent manganese, and 0.30 to 0.35 percent carbon With the balance essentially cobalt.

3. A metal alloy capable of being drawn in continuous wire lengths comprising, by Weight, from 27 to 30 percent chromium, a minimum of 56 percent cobalt, from 3 to 6 percent tungsten, from 2 to 4 percent nickel, from 0.03 to 0.2 percent manganese and from 0.25 to 0.40 percent carbon.

4. A metal alloy capable of being drawn in continuous wire lengths comprising, by weight, 28% chromium, 4.5% tungsten, 2.0% nickel, 0.15% manganese and 0.30 to 0.35% carbon with a balance essentially cobalt.

References Cited in the file of this patent UNITED STATES PATENTS 2,036,496 Randolph Apr. 7, 1936 2,097,768 McCurdy Nov. 2, 1937 2,299,860 Stoody Oct. 27, 1942 2,381,459 Merrick Aug. 7, 1945 2,392,510 Stoody Jan. 8, 1946 2,486,576 Savage Nov. 1, 1949 FOREIGN PATENTS 423,562 Great Britain Feb. 4, 1935 454,881 Great Britain Nov. 15, 1935 602,244 Great Britain May 24, 1948 OTHER REFERENCES Metals Progress, August 1939, pages 131-137, 180.

Materials and Methods, September 1953, No. 260, page 139.

Product Engineering, 1953 Annual Handbook, pages B1 2 

3. A METAL ALLOY CAPABLE OF BEILNG DRAWN IN CONTINUOUS WIRE LENGTHS COMPRISING, BY WEIGHT, FROM 27 TO 30 PERCENT CHROMIUM, A MINIMUM OF 56 PERCENT COBALT, FROM 3 TO 6 PERCENT TUNGSTEN, FROM 2 TO 4 PERCENT NICKEL, FROM 0.03 TO 0.2 PERCENT MANGANESE AND FROM 0.25 TO 0.40 PERCENT CARBON. 